Recent Posts

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21
I finally got my first DRSSTC up and running and I'm very happy that it's working, but there are still some issues.
https://youtube.com/shorts/LW7HfMb8ZDw?feature=share

The OCD is currently set to around 350A but it gets triggered when the input voltage reaches around 130VAC yet the sparks I'm getting are still very short. To check if this was an issue with my tuning or not, I removed the secondary and replaced it with a solid piece of metal as the load on top of the primary coil. The OCD still got triggered at around 130VAC input.

I scoped the current waveform using a voltage probe across the 51ohm resistor and got following waveform. The peak voltage is 23.2V which translates to 284A in the primary (I'm using 1:625 reduction CT) Because the primary current looks good I thought I had just tuned it like shit. But I had calculated the secondary resonant frequency using a function generator and an oscilloscope and it came to be 153kHz. So I adjusted the primary tap till I could see the primary current having a frequency of approximately 143kHz (Detune of 10kHz) but still I was not impressed with my arc output. And I couldn't increase the input voltage cause then the OCD LED would be constantly on.


Then I proceeded to scope the comparator inputs for the OCD. The blue waveform is the set point, currently set to approx 2.8V which translates to 343A as the limit. The yellow waveform is the other input of the comparator coming from the feedback. Both of the waveforms have more noise than usual because I had to attach jumper wires to the header J9 so I can easily probe these voltages. The waveform is just when the OCD got triggered.


I'm not sure what the issue is, one possibility is that the ferrite cores I'm using for the CT are not good, as the N30 cores I had ordered were too small, so I had to use some other cores I had laying around. But I had tested them by making a GDT with them and connecting them to the output of the UD2.7C output while using a function generator as the feedback. The output waveforms looked good so I assumed the cores were good enough to use for the CT. (The core for the final GDT is big and of N30 material so that's not the issue)

Other than that I think I've done everything properly, maybe the primary tap is a bit janky but it hey it works. I've attached images so maybe you can figure out if I've done something wrong.
22
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: DRSSTC Questions
« Last post by alan sailer on March 26, 2024, 08:46:59 PM »
Ionizing the air creating a path to the primary. Interesting idea.

All of the strike rings I have seen are smooth rings of metal. Another protective device is a lightening rod, which
uses a sharp point to create a potential concentration which directs the lightening return strike to a ground rod.

The only reason I can think of that this effect is not used in tesla coils is that it would encourage secondary to
ground arcs.

Cheers.
23
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: DRSSTC Questions
« Last post by flyingperson23 on March 26, 2024, 05:02:18 PM »
Ground strike rings are pretty good at keeping the arc from hitting the primary if properly placed. If it's too close, the arc to the ground ring can ionize the air in between the ground ring and primary and cause an arc there. I've heard of decoupling caps being useful for smaller coils, and also them doing nothing for large coils, so I'm not sure for that.
24
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: DRSSTC Questions
« Last post by alan sailer on March 26, 2024, 03:16:03 PM »
Thanks for the ideas. I will raise the top load a bit, make a longer breakout and reduce the coupling by raising the secondary.
These all should move the top load away from the primary/strike ring.

I have done no real tuning as of yet. I have to finish getting the coil into it's final mechanically stable condition.

I am curious if anyone has an idea how effective strike rings and that de-coupling cap on the bridge are?
Do they prevent bridge death or just make it less likely?

Once again, the help is very useful.

Cheers.
25
Electronic Circuits / Re: CM400 Induction Heater
« Last post by Anders Mikkelsen on March 26, 2024, 01:41:49 PM »
Quote
I'm trying to wrap my head around why higher frequency is more effective on nonmagnetic loads where we do not have hysteresis losses - is it because higher frequency leads to a higher voltage across the work coil?
Yes, higher work coil voltage, which translates to higher volts/turn at load too.

Quote
With a steel tube in the coil, you might end up with a Q as low as 4, which allows you to transfer 34/4 = 8.5 kW while staying within the ratings of your capacitors. For solid steel well matched to the size of the coil, Q might be 5 - 8, up to a few times that for smaller work pieces. For aluminium workpieces, a Q in the 20 - 50 range is not unrealistic, and for copper it can exceed 100.
Anders:  Any idea what Q range would be for graphite crucibles?  I'd expect Q to be well lower than for directly heating copper or aluminum.  If I understand correctly, Markus' plan is to use graphite crucibles for melting.

I just did some quick measurements with a tight fitting coil around a large crucible, the best practical case, and it goes from around 3.5 at 30 kHz to 5.5 at 100 kHz, so not too bad. This is practically with no clearance between the coil and crucible, so I would target maybe 5 - 8 Q at 50 kHz to give more flexibility in the crucible selection in practice. For a project of this cost and magnitude, I would not skimp on the tank VARs, and the numbers I gave for the FKP1s are really pushing the limit. It's also nice to be able to heat aluminum directly in ceramic crucibles, and steel above the curie point, since my experience is that graphite crucibles oxidize away with time if no protective atmosphere is used. I would still stand by my recommendation to use a purpose-made induction heating cap, given that a 300 kVAR part costs less than 100 euros from normal distributors, and 500 kVAR can be had for less than 150. This gives a lot more VARs per dollar (or euro) compared to FKP1s, with easier mounting, cooling and lower stray inductance to boot. I have  a good stock of new Celem 500 kVAR caps that I got for 25 dollars each, and I would be fine with donating one to the cause if Markus is in Europe, given the cost of shipping here.

Quote
I'm trying to wrap my head around why higher frequency is more effective on nonmagnetic loads where we do not have hysteresis losses - is it because higher frequency leads to a higher voltage across the work coil?
Yes, higher work coil voltage, which translates to higher volts/turn at load too.

Hysteresis loss is not usually very significant, and the large change in Q when steel goes above the curie point is mainly from the drop in permeability. Reflected resistance is proportional to the root of the ratio of resistivity to permeability. Hysteresis loss might represent some 15 - 30 % of losses, but the permeability can easily change from 1000 to 1, giving a loss ratio of sqrt(1000/1) = 30 between being below and above the curie point.

Inductive reactance of the coil, and therefore VARs per amp, rises linearly with frequency. Workpiece deposited power grows with the square root of frequency for a given coil and current, due to the skin depth decreasing with sqrt(F), so you get more heating per amp at higher frequency with a given coil and workpiece, but less heating per VAR. More coil turns gives more heating per amp, while preserving VARs, if the geometry is kept the same, but there's a practical upper limit to how many turns you can have. Since most film caps come with a 500+ V rating, it makes sense to dimension the tank to benefit from that.
26
You can see the tap point here.


I've mostly finished rebuilding the coil now. Hopefully this aluminum block will be enough heatsinking for the rectifiers.








Making things fit.


I put a few extra CTs on the bridge outputs to check current sharing and it appears to be identical on 2 separate bridge outputs!




27
General Chat / Re: Re-chargeable 1.5 volt lithium ion AAA batteries
« Last post by MRMILSTAR on March 26, 2024, 04:16:37 AM »
Thanks. I'll give that brand and charger a try if I can find them.
28
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: DRSSTC Questions
« Last post by davekni on March 26, 2024, 04:16:24 AM »
Quote
Other than raising the top-load I'm not sure what to do about this.
Looks like raising top-load a bit might be good for another reason, to avoid shorting upper turns of secondary.  A bit of gap between top of secondary and top load allows secondary magnetic field to exit top and return to bottom.  Will reduce secondary frequency a bit, so may need retuning.
29
Quote
https://protectglobal.com/products/fluid-containers/protect-foqus-600i-1100i-fluid-container/
Interesting!  I hadn't heard of "smoke" machines being used for security before.
30
Electronic Circuits / Re: CM400 Induction Heater
« Last post by davekni on March 26, 2024, 04:00:43 AM »
Quote
I'm trying to wrap my head around why higher frequency is more effective on nonmagnetic loads where we do not have hysteresis losses - is it because higher frequency leads to a higher voltage across the work coil?
Yes, higher work coil voltage, which translates to higher volts/turn at load too.

Quote
With a steel tube in the coil, you might end up with a Q as low as 4, which allows you to transfer 34/4 = 8.5 kW while staying within the ratings of your capacitors. For solid steel well matched to the size of the coil, Q might be 5 - 8, up to a few times that for smaller work pieces. For aluminium workpieces, a Q in the 20 - 50 range is not unrealistic, and for copper it can exceed 100.
Anders:  Any idea what Q range would be for graphite crucibles?  I'd expect Q to be well lower than for directly heating copper or aluminum.  If I understand correctly, Markus' plan is to use graphite crucibles for melting.
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